Remove `AssignmentProviderConfig` and use parameters from `HostConfiguration` instead (#3181)

This PR removes `AssignmentProviderConfig` and uses the corresponding
ondemand parameters from `HostConfiguration` instead. Additionally
`scheduling_lookahead` and all coretime/ondemand related parameters are
extracted in a separate struct - `SchedulerParams`.

The most relevant commit from the PR is [this
one](https://github.com/paritytech/polkadot-sdk/pull/3181/commits/830bc0f5).

Fixes https://github.com/paritytech/polkadot-sdk/issues/2268

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Co-authored-by: command-bot <>